Those practiced in the art of chemistry are often confronted with the problem of selectively hydrogenating one olefinic double bond in the presence of another olefinic double bond. Two cases are possible:
i) where the two olefinic double bonds are in the same molecule (intramolecular), and PA1 ii) where the two olefinic double bonds are in separate molecules of a mixture (intermolecular).
Whenever the sites of two olefinic double bonds differ in some way such as the steric or electronic environment around the bond, a selective hydrogenation is theoretically possible. For example, the ease of hydrogenation of olefins is in the order: monosubstituted&gt;disubstituted&gt;trisubstituted&gt;tetrasubstituted. It should be possible, in theory, to hydrogenate the less substituted olefinic double bond in the presence of the more substituted bond. In practice, however, very few catalysts have been found that allow for such selective hydrogenations. P. N. Rylander et al. disclosed ruthenium on carbon as a selective heterogeneous catalyst in the hydrogenation of simple hydrocarbon mixtures (case ii above) [J. Org. Chem. 24, 708 (1959); U.S. Pat. No. 2,944,094]. While mono-substituted olefins could be hydrogenated selectively in the presence of di- and trisubstituted olefins, no selectivity was observed when an attempt was made to hydrogenate disubstituted olefins in the presence of trisubstituted olefins.
Those practiced in the art are often faced with the problem presented in case (i) above, i.e., the selective hydrogenation of one of the two double bonds in a diene, especially in dienes having other functional groups. This case can be illustrated by Scheme I where it is desirable to obtain the substituted 2-octene II (reaction 1) relatively free of the octadiene, I, and the octane III (reaction 2). (The group X can represent a number of functional groups, e.g., OH, OCH.sub.3, COOH, etc.) ##STR1##
In such cases, it is important to achieve good selectivity for the formation of II at a high conversion of I because the boiling points of I, II and III are usually so similar that it is not commercially feasible to separate II from I and III by distillation. Thus, the purity of II obtained after distillation will essentially be whatever the purity of II is in the crude mixture of I, II and III.
There have been a number of attempts to improve the selectivity in Scheme I in favor of the olefin II, by converting I to II under conditions that favor the formation of II at high conversion of I, without at the same time converting II to III. All of these attempts dealt with the use of homogeneous catalyst systems. For example, Tsuji et al. [Bull. Chem. Soc. Japan, 49, 1701, (1976)] found that the homogeneous catalyst RhCl(PPh.sub.3).sub.3 provided an 80% selectivity of II at 90% conversion of the octadiene I wherein X is an acetoxy group. This degree of selectivity, however, gives a product which is only about 72% pure. (For Scheme I, the purity of II can be calculated as the product of the selectivity of II times the conversion of I.) As has been suggested above, distillation will not provide a product where the content of II is increased because of the closeness of the boiling points. (The catalysts used by Tsuji et al. also suffer from the disadvantage that they are too slow unless pressures higher than 50 psi are used, e.g. 150-435 psi.)
The catalyst RuCl.sub.2 (PPh.sub.3).sub.3, also a homogeneous catalyst, was reported to be selective [J. Tsuji & H. Suzuki, Chem. Lett. 1083, 1977] but no data was given. (We have found that this catalyst gives product mixtures with a purity of 63% to 64%. See Examples 1 and 2.)
While the prior art systems give good selectivity, the selectivity is not sufficient to provide purities of 90 to 100% of II. When purities of II of from 90% to 100% are required or desirable, they will only be achieved with use of a catalyst that will allow II to be prepared in high selectivity as the conversion of I approaches 100%.